1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to a method and an apparatus for determining Time Division Duplex (TDD) configurations applicable for radio frames.
2. Description of the Related Art
In wireless communication systems, such as those defined by Third Generation Partnership Project (3GPP) Long Term Evolution (LTE/LTE-A) specification, User Equipments (UEs) and a base station (i.e., eNodeB (eNB)) communicate with each other by sending and receiving data carried in radio signals according to a predefined radio frame format. Typically, the radio frame format contains a sequence of radio frames, each radio frame having a plurality of sub frames. Typically, each radio frame has a duration of 10 milliseconds (ms) and consists of two half frames, each having a duration of 5 ms. Each half frame consists of 5 sub frames having a duration of 1 ms. Each sub frame consists of two slots, each having a duration of 0.5 ms. Each slot consists of a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols.
Typically, subframes in a radio frame are configured to perform Uplink (UL) transmission and Downlink (DL) reception in different duplexing methods. For example, a TDD mode is a duplexing method to separate transmitting and receiving radio signals in the time domain such that the same carrier frequency is used for transmission and reception. LTE release 8 specification (TS 36.300) specifies seven different TDD configurations to support different DL/UL traffic ratios for different frequency bands when the TDD mode is employed to perform UL transmission and DL reception. In each TDD configuration, each Subframe (SF) of a radio frame is defined as an Upink subframe (U), a Downlink subframe (D) or a Special subframe (S). For example, seven TDD configurations are illustrated in Table 1 below:
TABLE 1Config-Switch-pointSubframe numberurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
During an uplink subframe, the UE operates in a transmission mode while the eNB operates in a reception mode. During a downlink subframe, the UE operates in a reception mode while the eNB operates in a transmission mode. During a special subframe, the eNB switches from transmission mode to reception mode and the UE switches from reception mode to transmission mode.
At any given instance, any of above seven TDD configurations may be configured for a serving cell. In LTE Release 12 specification, dynamic TDD reconfiguration on a cell level is envisioned by adapting TDD configuration according to a prevailing traffic load situation in UL and DL for a concerned cell. The dynamic TDD reconfiguration of TDD configuration on a cell level allows optimum utilization of radio resources matching traffic data volume in UL and DL directions.
In Release 11, 3GPP Radio Access Network (RAN) Working Group 1 evaluated performance of dynamic TDD reconfiguration on time scale of 10 ms (fast), 200 ms (medium) and 640 ms (slow) in TR 36.828. The fast adaptation rate of 10 ms reconfigures TDD configuration on a radio frame basis. The slow adaptation rate of 640 ms reconfigures TDD configuration according to minimum System Information (SI) change modification period. For faster reconfiguration, a currently applicable TDD configuration is notified to a UE through physical layer signalling via a new Downlink Control Information (DCI) format. The new DCI format consists of code points to index some or all seven TDD configurations. Typically, a DCI format indicating new TDD configuration is transmitted on a Packet Data Control Channel (PDCCH), where the UE uses a System Information Radio Network Temporary Identifier (SI-RNTI) to decode the DCI format in the common search space of PDCCH.
In a radio frame, subframes SF#0 and SF#5 are fixed as DL subframes, SF#1 is fixed as special subframe, and SF#2 is fixed as UL subframe. For 5 ms switching periodicity configurations, subframe SF#6 is fixed as DL subframe while subframe SF#7 is fixed as UL subframe. Hence, subframes SF#3, SF#4, SF#8 and SF#9 are subframes available for flexible reconfiguration as DL or UL.
According to LTE Release 8 specification (TS 36.300), in order to enable reasonable battery power consumption at the UE, a Discontinuous Reception (DRX) cycle is employed per UE in a connected state. Discontinuous reception allows UE to turn off its radio transceiver during various periods in order to save battery life on the UE. When a DRX cycle is configured for a UE, the UE switches between on duration (also referred to as a period of activity) and off duration (also referred to as a period of inactivity). During the on duration period, the UE monitors one or more subframes of a radio frame. Upon expiry of the on duration period, the UE switches to sleep mode during the off duration period to conserve battery consumption.
When the eNB adapts a dynamic TDD reconfiguration to match a traffic load situation, the TDD UL-DL configuration may need to be reconfigured for each radio frame. As described above, the eNB transmits a new DCI format indicating the current TDD UL-DL configuration to the UE on subframes SF#0 and SF#5 in each radio frame so that the UE knows the TDD UL-DL configuration for each radio frame. However, the UE may be operating in an off duration of the configured DRX cycle when the eNB transmits a new DCI format indicating the current TDD UL-DL configuration applicable for the current radio frame in the subframes SF#0 and SF#5 of the current radio frame. As a consequence, the UE may not be aware of a correct TDD UL-DL configuration applicable for the current radio frame, causing the UE to go out of synchronization with the eNB.